Need a sharp lens around 400mm

I agree with schmoelzel.
I have the 100-400L and it's a very versatile lens.
However for birds and airshows you normally need reach.
Therefore if you have the light the 400 5.6 prime would be ideal.
I chose the 300f4 IS because the light tends to be a bit dull on this side of the Atlantic.
I added a 1.4 TC, giving me 420 f5.6 and I think the image is sharper than the 100-400 at 400.

The IS is a great bonus.
In fact it has made the difference between getting a shot and getting a nice Gaussian Blurr effect.

Oh .....I don't know
The Prime 400mm seems so so sharp and is lighter and cheaper than the zoom.
BUT the zoom had IS and more use in other situations???
Oh I think the zoom will still be the best bet for me.
But...............

Yes, the zoom is more versatile, however you said it was for birds and airshows. You will be at the 400 end of the zoom all the time.
I'll make a bet if you buy the zoom you will, at a later date, buy the 300 or 400 prime.
£5...............or is it $5...

Simon,
If it's any help to you, I was in the same boat as you a few months ago. I bought the 100-400L thinking I may very well end up with the 400L 5.6. I was leaning towards the prime, but got such a good deal on the zoom I decided to try it first. I quickly fell in love with the 100-400 and haven't given the prime a second thought. However, you stressed birds and for small birds I just haven't seen the sharpness as consistently with the zoom as with the prime. That's not to say I haven't seen some impressively sharp bird photos with the zoom, just not as consistently--maybe it takes a little PS sharpening. For larger wildlife I think the zoom is fine.

You might need to borrow Six Million Dollar Man's arm to hold the monster. But wait, didn't he have a normal left arm. Darn!

Redbird_xo wrote:
You might need to borrow Six Million Dollar Man's arm to hold the monster. But wait, didn't he have a normal left arm. Darn!

No doubt.

Not seen any really impressive Canon prime photographs? Spend a few dollars on "Art of Bird Photography" by Arthur Morris and then tell me you're not considering mortgaging your left kidney to get a 600mm f/4.

Thank you all for this very informative insight .
I will keep you informed.

I love this. It could go on for days.

condyk

I was speaking more along the lines of comparable quality lens, not good glass vs crap glass.
A zoom has more mechanics and glass between your film/sensor and the objects so therefore there maybe "some" degradation.

You should try to rent both a 100-400 and a 400 prime and see which one you prefer. The advantage of the the 100-400 is the IS.

I also read that both the new Tamron 200-500 and the Sigma 50-500 were sharper than the 100-400 at 400mm but they do not have IS. The 100-400 has the advantage of a zoom for easy framing and IS. A good deal to me. Hope this helps.

I think i will end up with the zoom in the end. If I become rich in the near future then i will buy a big prime too:p.
Just as long as it has IS though.........